1. Field of the Invention
This invention relates to the production of heat energy from solid carbonaceous fuels, for example solid wastes.
2. Description of Related Art
In conventional deep bed proces for the production of heat energy from solid carbonaceous fuels, a deep bed is formed in which primary combustion air with optional additional steam and/or other additives or recycled gases are introduced at or near the bottom of the bed causing partial oxidation and gasification of the char in the lower part of the bed whilst in the upper part of the bed pyrolysis of the freshly introduced fuel yields pyrolysis gas and char which sinks lower in the bed to be subjected to the aforementioned gasification process, the mixture of gases thus produced from the deep bed being thereafter mixed with secondary combustion air and burnt to produce heat energy.
Such a process is described in our co-pending U.K. Patent Application No. 9511090.4 which claims a process for producing heat energy from solid carbonaceous fuels having relatively high ash contents which process comprises subjecting the fuel to pyrolysis and gasification in a bed of the fuel in a combustion chamber, the fuel being reacted with primary air together with steam and/or water vapour and/or recycled exhaust gas, the gas produced from the bed being thereafter subjected to secondary combustion, wherein the fuel bed is unsupported by a grate, the primary air being introduced into the fuel bed by means of one or more apertured tubes projecting into and/or through the bed, and the ash resulting from the pyrolysis and gasification being continually removed from the bottom of the combustion chamber in a particulate and unclinkered state.
Other processes are known in which carbonaceous fuels are conveyed through a substantially horizontal or inclined reaction chamber whilst being subjected to combustion, optionally with addition of steam, so that the fuel is progressively pyrolysed and gasified as it passes through the chamber.
However, in such processes there is no clear boundary between the zones in which the respective pyrolysis and gasification steps take place even though the process may be conducted in a stagewise fashion and a consequence of this is that it is difficult to control the two different steps which are in fact quite different chemical processes and which have different requirements, and also to control the secondary combustion step, and therefore there is a problem that the overall process cannot be managed as efficiently as it might be. It is desirable to be able to control these process steps independently and thereby to optimise the overall process, in particular to enable a wide range of fuels and wastes to be handled in the process, and in apparatus designed for the processing of a wide range of fuels or wastes.
Furthermore, of course, it is desirable that the separate controllability should be achieved in a compact manner by means of relatively simple robust apparatus without introducing complexities which might offset any advantages obtained by separately controlling the gasification and pyrolysis steps.
Additionally, of course, any such separate controllability should be achieved without loss, or with minimum loss, in the thermal efficiency of the overall process.
DE 4 327 320 A1 describes a process and apparatus for the thermal disposal of waste including an initial step in which the waste is degasified in a degasification tube. The degasification tube is heated indirectly by energy derived from a waste heat boiler which extracts heat from flue gases derived from secondary combustion of gaseous products from the waste.
When the term "air" is used in this specification it is of course intended that any gas or mixture of gases comprising oxygen and which will effect combustion is within the scope of the disclosure.